Body Mass Index (BMI) is a simple index of weight-for-height that is commonly used to classify underweight, overweight and obesity in adults. It is defined as the weight in kilograms divided by the square of the height in metres (kg/m2). For example, an adult who weighs 70kg and whose height is 1.75m will have a BMI of 22.9.

BMI = 70 kg / (1.75 m2) = 70 / 3.06 = 22.9

Table 1: The International Classification of adult underweight, overweight and obesity according to BMI

Classification

BMI(kg/m2)

Principal cut-off points

Additional cut-off points

Underweight

<18.50

<18.50

Severe thinness

<16.00

<16.00

Moderate thinness

16.00 - 16.99

16.00 - 16.99

Mild thinness

17.00 - 18.49

17.00 - 18.49

Normal range

18.50 - 24.99

18.50 - 22.99

23.00 - 24.99

Overweight

≥25.00

≥25.00

Pre-obese

25.00 - 29.99

25.00 - 27.49

27.50 - 29.99

Obese

≥30.00

≥30.00

Obese class I

30.00 - 34.99

30.00 - 32.49

32.50 - 34.99

Obese class II

35.00 - 39.99

35.00 - 37.49

37.50 - 39.99

Obese class III

≥40.00

≥40.00

Source: Adapted from WHO, 1995, WHO, 2000 and WHO 2004.

BMI values are age-independent and the same for both sexes. However, BMI may not correspond to the same degree of fatness in different populations due, in part, to different body proportions. The health risks associated with increasing BMI are continuous and the interpretation of BMI gradings in relation to risk may differ for different populations.

In recent years, there was a growing debate on whether there are possible needs for developing different BMI cut-off points for different ethnic groups due to the increasing evidence that the associations between BMI, percentage of body fat, and body fat distribution differ across populations and therefore, the health risks increase below the cut-off point of 25 kg/m2 that defines overweight in the current WHO classification.

There had been two previous attempts to interpret the BMI cut-offs in Asian and Pacific populations3,4, which contributed to the growing debates. Therefore, to shed the light on this debates, WHO convened the Expert Consultation on BMI in Asian populations (Singapore, 8-11 July, 2002)5.

The WHO Expert Consultation5 concluded that the proportion of Asian people with a high risk of type 2 diabetes and cardiovascular disease is substantial at BMI's lower than the existing WHO cut-off point for overweight (= 25 kg/m2). However, the cut-off point for observed risk varies from 22 kg/m2 to 25 kg/m2 in different Asian populations and for high risk, it varies from 26 kg/m2 to 31 kg/m2 . The Consultation, therefore, recommended that the current WHO BMI cut-off points (Table 1) should be retained as the international classification.

But the cut-off points of 23, 27.5, 32.5 and 37.5 kg/m2 are to be added as points for public health action. It was, therefore, recommended that countries should use all categories (i.e. 18.5, 23, 25, 27.5, 30, 32.5 kg/m2 , and in many populations, 35, 37.5, and 40 kg/m2) for reporting purposes, with a view to facilitating international comparisons.

Discussion updates
A WHO working group was formed by the WHO Expert Consultation5 and is currently undertaking a further review and assessment of available data on the relation between waist circumference and morbidity and the interaction between BMI, waist circumference, and health risk.